System for stripping and rectifying a fluid mixture

ABSTRACT

A heat exchanger is used between the stripper section and rectifier section. The heat exchanger ( 10 ) has channels closed on all sides with an end inlet and end outlet for the related section. A compressor is fitted between the sections. According to the invention the heat exchanger is a so-called compact heat exchanger, that is to say a heat exchanger made up of a series of adjacent plates ( 15, 16 ) between which channels ( 11, 12 ) closed on all sides for the related section are delimited. These channels ( 11, 12 ) extend essentially vertically and are so constructed that all liquid contained therein is able to move towards the bottom of the section concerned. The flow cross-section of the channels of the stripper/rectifier section changes depending on the process carried out therein.

[0001] The present invention relates to a system for stripping andrectifying a fluid mixture, comprising a heat exchanger with channelsclosed on all sides with end inlet and end outlet for a stripper sectionand a rectifier section, a compressor being fitted between the inlets ofsaid stripper section and rectifier section, said heat exchanger beingmade up of a series of adjacent plates between which channels closed onall sides for said stripper section or rectifier section are alternatelydelimited, and said channels extending essentially vertically and beingso constructed that all liquid contained therein is able to move towardsthe bottom of said section.

[0002] A system of this type is disclosed in U.S. Pat. No. 5,592,832.The so-called Heat Integrated Distillation Column process is describedin this document. This is a process where a compressor is fitted betweenthe stripper section and rectifier section. Moreover, direct heatexchange takes place between the rectifier section and stripper section,that is to say there is direct heat transfer from the rectifier sectionto the stripper section in a shell-and-tube heat exchanger. As a result,condenser and reboiler can be made of relatively small size. Theefficiency from conventional distillation can be improved in this way.The pressure difference generated as described in U.S. Pat. No.5,592,832 is associated with a corresponding difference in operatingtemperature, as a result of which heat can be transferred directly fromthe rectifier section to the stripper section.

[0003] A system of this type is used, for example, for separatinghydrocarbons having boiling points which are close to one another, suchas, for example, propane and propene. The mixture is fed to the top ofthe stripper section and, on the one hand, falls through the strippersection and, on the other hand, is pumped by the compressor to the inletof the rectifier section, which is at the bottom of the verticallyarranged rectifier section. Packing can optionally be present in thevarious sections. During operation there are relatively large gas andliquid streams in the top of the stripper section and minor gas andliquid streams at the bottom. The situation is reversed for therectifier section. There are large gas and liquid streams at the bottom,that is to say the inlet, and minor gas and liquid streams at the top.

[0004] As a result of this arrangement it is possible usefully to usethe relatively high value heat from the rectifier section in thestripper section. This is in contrast to prior processes where heat thatwas of relatively low value was recovered exclusively in the condenserand where relatively high value heat was exclusively supplied to thereboiler. The process yield can be increased as a result.

[0005] A heat exchanger of the plate/fin type with two groups ofpassageways for rectifying and stripping is described in U.S. Pat. No.5,592,832. This heat exchanger consists of a number of verticallyextending plates/fins arranged in parallel alongside one another. A HeatIntegrated Distillation Column (HIDiC) process is also described in EP 0726 085 A1.

[0006] An aim of the present invention is to provide a system with whicha heat exchanger can be implemented in a very compact manner. The heatand mass transfer can be optimised as a result of the compactconstruction.

[0007] This aim is achieved with a system as described above in that theflow cross-section of the channels of said section decreases from theinlet end to the outlet end.

[0008] Taking account of the liquid and gas streams which are present ina section in various locations in different states, according to theinvention the flow cross-section of the channels of one or both sectionsis constructed such that it become smaller from the inlet towards theoutlet. After all, the gas and liquid stream decreases towards theoutlet, as a result of which a smaller volume is needed. By positioningthe channels of the rectifier section and stripper section alongside oneanother it is possible to make optimum use of the space between theplates. With this arrangement it is, however, necessary that measuresare taken for sealing the sections with respect to one another.

[0009] If the counter-flow principle is used, that is to say the outletis below the inlet in the stripper section whilst the outlet is abovethe inlet in the rectifier section, convergence of adjacent channels canbe implemented such that the axes thereof remain parallel.

[0010] According to a further preferred embodiment, the change incross-section of the related channels is obtained by changing thedistance between two opposing walls of the heat exchanger. The otherwalls (end plates) of the heat exchanger are parallel to one another.

[0011] A compact heat exchanger of this type can be produced by anymethod known from the state of the art. According to an advantageousembodiment, the channels are recessed in the plates and in particularare made therein by etching. Furthermore, it is possible to make thechannels in one or more plates and to join these plates to one anotherin some manner known in the state of the art. In particular diffusionbonding is used for this purpose.

[0012] Compact heat exchangers have a larger heat exchange surface areaper unit volume than shell-and-tube heat exchangers. As a result theinstallation concerned can be made more compact and the temperaturedifferences can be limited, as a result of which the pressure differencecan be reduced and thus a smaller and less expensive compressor can beused and the yield increases.

[0013] According to a further advantageous embodiment, it is possible tocoat either the plates or added parts described above with, for example,catalyst material or to introduce such a catalyst material or some otherreactant in granular form into the channels concerned.

[0014] Compact heat exchangers can easily be made of modularconstruction, as a result of which modifications to suit changingconditions can easily be made. Furthermore, it is possible to adjust theratio between heat transfer surface area and mass transfer surface area,which is critical for a good design, in a particularly simple manner byadjusting the geometry of the compact heat exchanger. This applies inparticular for the embodiment described above, where fins, for exampleextending in a corrugated shape, are arranged between the parallelplates. By adjusting the number and/or the spacing, the ratio betweenthe heat transfer and mass transfer can be influenced.

[0015] The system described above can be used for stripping/rectifying awide variety of types of substances. The various aspects are notrestricted to hydrocarbons or propane/propene.

[0016] The compact heat exchanger can be constructed in a wide varietyof ways. In general each vertical channel will be provided with a liquidinlet and gas outlet above the heat-exchanging surface and a gas inletand a liquid outlet below the heat-exchanging surface. Moreover, theseinlets and outlets of each channel must be arranged such that these canbe connected to corresponding parts of other channels with the aid of abox or manifold. The separation between gas and liquid stream does notnecessarily have to take place in each channel, but can also be carriedout outside this. All that is important is that the incoming medium isuniformly distributed over the various channels so that optimum heatexchange and mass transfer can be provided. If appropriate, specialmeasures can be taken for this purpose. This applies both at the gasoutlet and liquid inlet and at the gas inlet and liquid outlet.

[0017] The invention will be explained in more detail below withreference to illustrative embodiments shown in the drawing. In thedrawing:

[0018]FIG. 1 shows the general functioning of the HIDIC concept;

[0019]FIG. 2 shows a compact heat exchanger constructed as plate/finheat exchanger;

[0020]FIGS. 3a and 3 b show the shape of channels in a particularcompact heat exchanger; and

[0021]FIG. 4 shows an alternative for the construction in FIGS. 2 and 3.

[0022] A method for separating two components is shown diagrammaticallyin FIG. 1. The mixture to be separated is fed to a stripper section S at1. The gaseous product is fed via a line to compressor 2 and compressedin a manner not shown in more detail and fed to rectifier section R. Theliquid product produced in this section is returned to line 1. Gas isfed to condenser 3.

[0023] Liquid that is produced in stripper section S is fed to areboiler 4, and then partially discharged as a bottom product. The heattransfer from the rectifier section to the stripper section is indicatedby the arrows 5. It will be understood that it is important to allowthis heat transfer to take place as efficiently as possible.

[0024] According to the invention this can be achieved with so-calledcompact heat exchangers.

[0025] An example of such a heat exchanger is given in FIG. 2, where acompact heat exchanger is indicated in its entirety by 10. This heatexchanger consists of alternating channels 11 and 12. There are fins 13and 14, respectively, which extend in a meandering fashion in each ofthe channels. These fins are so arranged that the channels always extendvertically. Plates 15 and 16, respectively, are always located betweenchannels 11 and 12.

[0026] As is clear from the drawing, the cross-sectional area of eachchannel changes from the bottom to the top. Channels 11 are part of thestripper section and have a cross-sectional area that increases from thebottom to the top. Channels 12 are part of the rectifier section andhave a cross-sectional area that decreases from the bottom to the top.There is counter-current flow in both types of channels, with gasflowing upwards and liquid flowing downwards. The end plates of thechannels, which are perpendicular to the heat-transfer plates 15 and 16,are parallel to one another. Channels and 11 and 12 can be provided withalternative fillings, such as structured packing, instead of fins, inorder to improve the mass transfer. Various types of structured packing,based on gauze or wire mesh or corrugated plates, are well known tothose skilled in the art of distillation.

[0027] The manifolds have not been included in this drawing.

[0028] Plates 24 and 25, which are arranged in contact with one anotherand in which channels 26 and 27, respectively, have been made, are shownin FIGS. 3a and b. As can be seen, these channels are ofconvergent/divergent construction. FIG. 3a relates to the strippersection where the cross-section is decreased towards the bottom becausea relatively smaller liquid and gas stream is present in this location.FIG. 3b shows a rectifier section where the channels are constructedprecisely the other way round. Instead of being conical, the channelscan also be made stepped or varying in cross-sectional dimension in someother way.

[0029] The channels can optionally be filled with a packing whichcontains a reagent or catalyst.

[0030] Variants for changing the cross-section of the channels are shownin FIGS. 4a and b. In these figures, in contrast to FIG. 3, the platesare not arranged in parallel. This is preferably a heat exchanger of theplate/fin type. In FIG. 4a the plates are inclined with respect to oneanother (as in FIG. 2) and in FIG. 4b the plates are of steppedconstruction.

[0031] Although the invention has been described above with reference toa few embodiments of a compact heat exchanger, it will be understood bythose skilled in the art that further modifications are possible.Reference is made to compact heat exchangers such as are described in“Learning from experience with compact heat exchangers” by Prof. DavidReay, published by the Centre for the Analysis and Dissemination ofDemonstrated Energy Technologies. Furthermore, reference is made to thepublication entitled “Compact heat exchangers” by J. E. Hesselgreaves,Pergamon, 2001. Heat exchangers can be in many different forms. Suchvariants fall within the scope of the present invention. An embodimentin which two parallel plates are wound together and which is indicatedwith spiral configuration may be mentioned as an example. The compressorcan be designed depending on the process. The same applies in respect ofthe size and presence of the condenser and/or reboiler 3 and 4,respectively, as shown in FIG. 1.

[0032] These and further modifications are obvious to those skilled inthe art after reading the above description and are within the scope ofthe appended claims.

1.-11. (cancelled)
 12. System for stripping and rectifying a fluidmixture, comprising a heat exchanger with channels closed on all sideswith end inlet and end outlet for a stripper section and a rectifiersection, a compressor being fitted between the inlets of said strippersection and rectifier section, said heat exchanger being made up of aseries of adjacent plates between which channels closed on all sides forsaid stripper section or rectifier section are alternately delimited,and said channels extending essentially vertically and being soconstructed that all liquid contained therein is able to move towardsthe bottom of said section, the flow cross-section of the channels ofsaid section decreasing from the inlet end to the outlet end.
 13. Systemaccording to claim 12, wherein said flow cross-section graduallychanges.
 14. System according to claim 12, wherein said flowcross-section decreases stepwise.
 15. System according to claim 12,wherein said channels comprise recesses in a plate.
 16. System accordingto claim 15, wherein said channels have been etched out of said plate.17. System according to claim 12, wherein the channels are delimited byplate sections placed on one another, which plate sections have beenfitted to one another by diffusion bonding.
 18. System according toclaim 12, wherein the outlet end of said rectifier section is connectedto a condenser.
 19. System according to claim 12, wherein the outlet endof said stripper section is connected to a reboiler.
 20. Systemaccording to claim 12, wherein said heat exchanger is a counter-flowheat exchanger and the cross-sections of both the stripper section andthe rectifier section decrease from the inlet to the outlet end thereof.21. System according to claim 20, wherein the axis of the strippersection channels is parallel to the axis of the rectifier sectionchannels.